The present invention relates to an electric power steering apparatus, in which motor power acts on a power steering system directly so that the steering force of a driver is reduced.
An electric power steering apparatus employs the driving force of a motor directly to assist the steering force of a driver. In addition to the motor, the electric power steering apparatus is equipped with a steering torque detection means for detecting steering torque acting on a steering system, a control means for controlling the motor and a motor drive means for driving the motor. The motor drive means is composed of a plural of FET""s (Field Effect Transistor) etc., which run the current of several tens of amperes in order to drive the motor to generate heat. If the control means is placed near the motor drive means, the reliability of the control means may possibly be degraded, since an internal analogue circuit to shape an inputted steering torque signal is affected by the heat.
The applicant disclosed in Japanese Utility Model Gazette 4-27743 an electric power steering apparatus of rack and pinion type, in which a control circuit (control means) and a torque detector were placed apart from a drive circuit (motor drive means) across a rack axis. The control circuit thus avoids the effect of heat of the drive circuit. However, this power steering apparatus employs a DC (Direct Current) brush motor as a motor, encountering a possible case of the brush abrasion due to the secular change. Once the brush is abraded, it may cause an increase in the voltage drop between the brush and the commutator or the unstableness of the motor electric current. Thus, the steering feeling of a driver may possibly be degraded due to the motor output drop or the unstableness of the motor electric current. Further, its moment of inertia is large, for the brush motor has a rotor including a steel core with winding. The large moment of inertia causes difficulty in handling a steering wheel for fine steering, since the moment of inertia acts on the steering wheel. It may possibly lead to such a problem as the difficulty of fine steering of a steering wheel on an expressway, the scratching feeling during steering back or the degradation of steering feeling.
An electric power steering which employs a brushless motor has been proposed. A brushless motor has a three-phase winding as an outer stator and a plurality of permanent magnets as an inner rotor. The three-phase winding is on-off controlled according to the rotation angle of the inner rotor, thus rotationally driving the inner rotor. A brushless motor is free from the degradation of steering feeling due to the brush abrasion mentioned above, for it needs no brushes. Also a brushless motor is immune to the degradation of steering feeling due to the large moment of inertia mentioned before, since magnets can be employed for the inner rotor of the brushless motor to reduce the moment of inertia.
The prior control unit of an electric power steering apparatus with a brushless motor will be described referring to FIG. 6. A control unit 100 outputs a motor control signal VO to a motor drive circuit 101 in order to PWM (Pulse Width Modulation) drive or turn off the six FET""s, which form the motor drive circuit 101 that drives a brushless motor 102. The control unit 100 sets a target current signal according to a steering torque signal T of a steering torque sensor TS and a vehicle speed signal V of a vehicle speed sensor VS in a target current setting sub-unit 100a, and also a compensation current signal accounting for inertia control and damping control with the steering torque signal T and the vehicle speed signal V in a compensation current setting sub-unit 100b. Further, the control unit 100 performs addition or subtraction of the compensation current signal for the target current signal in a compensator 100c. The control unit 100 generates the motor control signal VO according to the target current signal compensated in a drive control sub-unit 100d, a motor electric current signal IMO detected by a motor electric current detection means 103 and a motor rotation signal PMO detected by a motor rotation detection means 104. Then in the motor drive circuit 101, the FET""s start PMW driving according to the motor control signal VO and impose voltage VM on the three-phase winding of the brushless motor 102. In this connection, the motor drive circuit 101 generates heat since the FET""s run several tens of amperes. Therefore, when the brushless motor 102 is employed, the control unit 100 is also placed apart from the motor drive circuit 101 to prevent the degradation of reliability due to heat according to the arrangement disclosed in Japanese Utility Model Gazette 4-27743.
In the electric power steering apparatus according to Japanese Utility Model Gazette 4-27743, the signal flow (or supplied current flow) starting with the motor rotation signal PMO will be: the motor rotation detection means 104xe2x86x92the control unit 100xe2x86x92the motor drive circuit 101xe2x86x92the brushless motor 102, resulting in a long transmission path. Especially, the control unit 100 and the motor drive circuit 101 are placed apart each other to avoid the heat effect, and the transmission path between them is longer for that. Thus the signal starting with the motor rotation signal PMO takes time to pass through the transmission path, thereby involving phase lag. The brushless motor 102 requires an accurate on-off control for its drive according to the rotation angle of an inner rotor. However, in the arrangement of the electric power steering apparatus mentioned above, the signal starting with the motor rotation signal PMO involves phase lag. Thus the brushless motor 102 may not possibly be controlled accurately, since it is controlled by the rotation phase of the inner rotor including phase lag. As a result, the output of the brushless motor 102 oscillates or exhibits unstableness, which may possibly lead to hunting of the steering wheel or the degradation of steering feeling.
The object of the present invention is to provide an electric power steering apparatus, which is capable of controlling a brushless motor accurately and giving excellent steering feeling as well as maintaining the reliability of the control means.
The present invention to address the above issues provides an electric power steering apparatus, comprising: a brushless motor for adding assist torque to a steering system; a steering torque detection means for detecting steering torque acting on the steering system and outputting a steering torque signal; a motor rotation detection means for detecting a rotation of the brushless motor and outputting a motor rotation signal; a motor current detection means for detecting a motor current, which flows in the brushless motor, and outputting a motor current signal; a target current setting means for setting a target current based on at least the steering torque signal and outputting a target current signal; a drive control means for outputting a motor control signal based on at least the target current signal, the motor current signal and the motor rotation signal; and a motor drive means for driving the brushless motor based on the motor control signal, wherein the target current setting means and the drive control means are placed apart and electrically connected each other with a communication line.
The electric power steering apparatus according to the present invention enables the target current setting by the control means without heat effect, since the control means including parts, which should be free from heat effect, such as the analogue circuit for shaping the steering torque signal sent from the steering torque detection means, is placed apart from the drive means including the heat generating motor drive means. As the motor rotation signal enters the drive means in this electric power steering apparatus, the signal flow starting with the motor rotation signal is: the motor rotation detection meansxe2x86x92the drive meansxe2x86x92the motor, resulting in a short transmission path. Thus the electric power steering apparatus allows an accurate controlling of the motor (brushless motor) without phase lag in the signal starting with the motor rotation signal.
Further, the present invention provides the electric power steering apparatus, wherein the drive control means and the motor drive means are placed adjacent to the motor.
This electric power steering apparatus enables a short lead wire, which connects the motor drive means and the motor electrically, since the drive means is placed adjacent to the motor. Therefore, comfortable steering feeling can be attained by eliminating the voltage drop of the lead wire and the output drop of the motor while the motor is on. The shorter the lead wire is, the better it will be, when the drive means is placed adjacent to the motor. However, it may be acceptable if the drive means and the motor are placed close enough so that the effect of voltage drop of the lead wire is negligible, taking into account the size, the shape and the shape of mounting place of the drive means and the motor.